1. Field of the Invention
The present invention relates to a fog-resistant structure such as lenses for nearsighted glasses, farsighted glasses, and sunglasses; lenses for goggles for skiing and goggles for motorcycling; and shields for eye protection such as sports helmets and hard hats; and a protective device for eyes including the fog-resistant structure such as glasses, sunglasses, goggles, and helmet shield devices.
2. Description of the Related Art
Glass lenses and like fog in such a manner that heat generated from the face surface of the wearer causes a difference in temperature between the air contacting the lens inner surface facing the face surface and the external air contacting the lens outer surface, and water vapors in the air on the lens inner surface side condense on the lens inner surface.
In light of this, glass lenses are configured such that a transparent conductive film made of tin oxide, indium oxide, or the like is formed on the surface of the glass lenses, an electrode is fixed on upper and lower edges of the transparent conductive film, an electrode protection plate is fixed on the surface of each electrode, a power supply line is connected to each of the upper and lower electrodes, power is supplied from the power supply to the transparent conductive film for conduction, the lens temperature is increased to dry water vapors condensed on the lens inner surface, and thereby the glass lenses are prevented from fogging.
However, when such a glass lens configuration is applied to the goggle lenses for skiing and motorcycling or shields for sports helmets, the external temperature contacting the outer surface of the lenses or shields is extremely lowered by the high speeds during skiing and motorcycling to cause a significant difference in temperature between the face surface of the wearer and the lenses or shields. Therefore, a large power is needed to increase the temperature of the lenses or shields. Thus, there is a problem with wasted power consumption and large-sized power supply causing inconvenience of wearing.
In light of this, for example, Patent Literature 1 (Japanese Patent Laid-Open No. 50-147192) discloses a structure suitable for preventing fogging of goggle lenses for skiing and motorcycling or shields for sports helmets.
The structure disclosed in Patent Literature 1 is configured such that as illustrated in FIG. 9, a transparent conductive film 12 is formed on the entire surface of a lens 11, a linear electrode 13 and an electrode protection plate 14 are fixed to the upper and lower edges of the surface of the transparent conductive film 12 in this order, one end of a power supply line 15 is fixed to the upper and lower electrodes 13 or the upper and lower electrode protection plates 14, and the other end of the power supply line 15 is connected to a power switch. This structure is such that a spacer 16 made of an elastic, cold-resistant, and heat-resistant material and a separate lens 17 on the other surface of the spacer 16 are fixed to the transparent conductive film 12 side in this order, and a sealed empty space 18 is interposed between the transparent conductive film 12 and the separate lens 17.
However, even for the goggle lenses for skiing and motorcycling or shields for sports helmets, the temperature in a central region of a lens or shield is relatively lowered and the temperature in opposite side regions of the lens or shield is relatively increased, thus causing a difference in temperature in those regions.
The reason for causing such a difference in temperature is that each portion of the transparent conductive film having electrical resistance has different spacing between linear electrodes. More specifically, in opposite side portions of the lenses or shields, the spacing between linear electrodes is small and thus the electrical resistance is low; current is easy to flow and thus the amount of heat generation increases relatively, while in a central region of a lens or shield, the spacing between linear electrodes is large and thus the electrical resistance is high; current is difficult to flow and thus the amount of heat generation decreases relatively.
In order to secure the field of view of the helmet wearer, first, the temperature is increased to prevent fogging in the central portion of the lens or shield. However, when an electric current is applied to increase the temperature of the central region to a degree required to prevent fogging, an electric current higher than that thereof flows through the regions of opposite side portions, thereby causing wasted power consumption. In particular, a structure using a battery as the power supply such as helmet shields has a problem in that the available time of the anti-fog heater is very limited.
In order to solve such a problem, for example, Patent Literature 2 (Japanese Patent Laid-Open No. 2002-237371) discloses a helmet shield.
According to Patent Literature 2, as illustrated in FIG. 10, each of the linear electrodes 23 and 24 is divided into three split linear electrodes 23a to 23c and 24a to 24c respectively according to the difference in length in the conducting direction (vertical direction) in each portion of the transparent conductive film 22 heating the shield 21. These split linear electrodes are used for regulation such that an regulation is made on power supplied to the portions of the transparent conductive film 22 on opposite side regions 25 of the shield 21 and the portion of the transparent conductive film 22 on the central region 26 thereof so that each portion has substantially the same amount of heat generation or the central region 26 has a slightly larger amount of heat generation.
Unfortunately, the helmet shield disclosed in Patent Literature 2 has a problem in that in order to regulate power supplied to the transparent conductive film 22, a constant-voltage circuit having a plurality of output voltages needs to be interposed between the vertically divided linear electrodes 23a to 23c and 24a to 24c and the power supply or a power regulating controller needs to be provided, thereby complicating the structure and increasing the unit price of the product.
Further, the helmet shield disclosed in Patent Literature 2 has a problem in that in order to supply power to the transparent conductive film 22, power supply lines 27a to 27d need to be connected to the vertically divided linear electrodes 23a to 23c and 24a to 24c respectively, thereby complicating the wiring of the plurality of power supply lines 27a to 27d and causing an easy-to-fail and hard-to-maintain problem.